The present invention relates to a noise field intensity measuring apparatus, and more particularly to a noise field intensity measuring apparatus which is capable of measuring the noise field intensity over a wide range.
In conventional noise field intensity measuring apparatus, a noise wave is received by receiver changing the receiving frequency; an intermediate-frequency signal of the receiver is rectified by a rectifier; a DC component is taken out by a low-pass filter from the rectified output; and then converted by a time constant circuit into a quasi-peak value detected output; and the quasi-peak value detected output is applied to an indicator, such as a CRT display, a meter type indicator or the like, for display of its level. The measurement of such noise field intensity is standardized by the CISPR standard. In such a noise field intensity measuring apparatus, as the dynamic range of the indicator is narrow, the range over which the noise field intensity can be measured is narrow, only 0 to -20 dB or so in terms of input field intensity.
As a solution to this problem, a method that supplies the output from the time constant circuit to a DC logarithmic amplifier to compress the variation range of the input thereto and which applies the output therefrom to the indicator is described with reference to FIG. 5 of U.S. Pat. No. 4,272,719 "Electric Field Intensity Measuring Apparatus" issued on June 9, 1981. With such a method, the field intensity measuring range can be enlarged but, in this case, the dynamic range of the rectifier for rectifying the received intermediate-frequency output is about 40 dB and, under the restriction of such a narrow dynamic range, the field intensity measuring range is enlarged to 0 to -40 dBm only. Accordingly, for further enlargement of the dynamic range for measurement, it has been considered to compress the level of the output signal by an AC logarithmic amplifier at a stage preceding the rectifier while the signal still remains as an AC signal. However, as the AC logarithmic amplifier is complicated in structure and low in accuracy, it is difficult to measure the noise level with high accuracy. In addition, the AC logarithmic amplifier includes a feedback circuit, and hence is poor in response.
It is therefore an object of the present invention to provide a noise field intensity measuring apparatus of wide dynamic range for measurement.
Another object of the present invention is to provide a noise field intensity measuring apparatus which is wide in dynamic range, simple in construction, and inexpensive and excellent in accuracy and response.